Contrail
Encyclopedia
Contrails or vapour trails are artificial clouds that are the visible trails of condensed
water vapour made by the exhaust of aircraft
engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets or, if the air is cold enough, tiny ice
crystals.
The wingtip vortices
which trail from the wingtips and wing flaps
of aircraft are sometimes partly visible due to condensation in the cores of the vortices. Each vortex
is a mass of spinning air and the air pressure at the centre of the vortex is very low. These wingtip vortices are not the same as contrails.
Depending on atmospheric conditions, contrails may be visible for only a few seconds or minutes, or may persist for many hours which may affect climate.
Contrails tend to last longer if there is higher moisture in the atmosphere and associated higher level clouds such as cirrus, cirrostratus and cirrocumulus already present before the plane flies through.
are carbon dioxide
and water vapour. At high altitudes this water vapour emerges into a cold environment, and the local increase in water vapour can push the water content of the air past saturation point
. The vapour then condenses into tiny water droplets and/or deposits into ice. These millions of tiny water droplets and/or ice crystals form the contrails. The vapour's need to condense accounts for the contrail forming some way behind the aircraft's engines. At high altitudes, supercooled water vapour requires a trigger to encourage deposition or condensation. The exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapour to rapidly turn to ice crystals. Exhaust contrails usually occur above 8000 metres (26,000 feet), and only if the temperature there is below -40 °C.
, it causes a vortex
to form at each wingtip, and sometimes also at the tip of each wing flap
. These wingtip vortices
persist in the atmosphere long after the aircraft has passed. The reduction in pressure and temperature across each vortex can cause water to condense and make the cores of the wingtip vortices visible. This effect is more common on humid days. Wingtip vortices can sometimes be seen behind the wing flaps of airliners during takeoff and landing, and during landing of the Space shuttle
.
The visible cores of wingtip vortices contrast with the other major type of contrails which are caused by the combustion of fuel. Contrails produced from jet engine exhaust are seen at high altitude, directly behind each engine. By contrast, the visible cores of wingtip vortices are usually seen only at low altitude where the aircraft is travelling slowly after takeoff or before landing, and where the ambient humidity is higher. They trail behind the wingtips and wing flaps rather than behind the engines.
During high-thrust settings the fan blades at the intake of a turbo-fan engine reach transonic speeds, causing a sudden drop in air pressure. This creates the condensation fog (inside the intake) which is often observed by air travelers during takeoff. For more information see the Prandtl-Glauert singularity
effect.
. Studies have found that contrails trap outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) at a greater rate than they reflect incoming solar radiation (negative radiative forcing). Global radiative forcing has been calculated from the reanalysis data, climatological models and radiative transfer codes. It is estimated to amount of 0.012 W/m2 for 2005, with an uncertainty range of 0.005 to 0.0026 W/m2, and with a low level of scientific understanding. Therefore, the overall net effect of contrails is positive, i.e. a warming effect. However, the effect varies daily and annually, and overall the magnitude of the forcing is not well known: globally (for 1992 air traffic conditions), values range from 3.5 mW/m2 to 17 mW/m2. Other studies have determined that night flights are mostly responsible for the warming effect: while accounting for only 25% of daily air traffic, they contribute 60 to 80% of contrail radiative forcing. Similarly, winter flights account for only 22% of annual air traffic, but contribute half of the annual mean radiative forcing.
Condensation trails have been suspected of causing "regional-scale surface temperature" changes for some time. Researcher David J. Travis, an atmospheric scientist at the University of Wisconsin-Whitewater
, has published and spoken on the measurable impacts of contrails on climate change in the science journal Nature and at the American Meteorological Society
10th Annual conference in Portland, Oregon. The effect of the change in aircraft contrail formation on the 3 days after the 11th was observed in surface temperature change, measured across over 4,000 reporting station
s in the continental United States. Travis' research documented an "anomalous increase in the average diurnal temperature change". The diurnal temperature range
(DTR) is the difference in the day's highs and lows at any weather reporting station. Travis observed a 1.8 degree Celsius departure from the two adjacent three-day periods to the 11th-14th. This increase was the largest recorded in 30 years, more than "2 standard deviations away from the mean DTR".
, U.S.
, a contrail of this type gained wide media attention as a "mystery missile" that could not be explained by US military and aviation authorities, and its explanation as a contrail took more than 24 hours to become accepted by US media and military institutions.
A 'distrail' is short for dissipation trail. Where an aircraft passes through a cloud, it can clear a path through it; this is known as a distrail. Because the plane's contrail is not yet visible (contrails usually form above 26,000 feet, depending on the temperature and other factors) the distrail looks like a tunnel through the cloud if the cloud is very thin.
Distrails are created by the elevated temperature of the exhaust gases absorbing the moisture from the cloud. Clouds exist where the relative humidity
is 100% but by increasing the temperature the air can hold more moisture and the relative humidity drops below 100%, even for the same absolute moisture density, causing the visible water droplets in the cloud to be converted back into water vapour.
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
water vapour made by the exhaust of aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets or, if the air is cold enough, tiny ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
crystals.
The wingtip vortices
Wingtip vortices
Wingtip vortices are tubes of circulating air that are left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure...
which trail from the wingtips and wing flaps
Flap (aircraft)
Flaps are normally hinged surfaces mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed an aircraft can be safely flown at and to increase the angle of descent for landing without increasing air speed. They shorten takeoff and landing distances as well as...
of aircraft are sometimes partly visible due to condensation in the cores of the vortices. Each vortex
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...
is a mass of spinning air and the air pressure at the centre of the vortex is very low. These wingtip vortices are not the same as contrails.
Depending on atmospheric conditions, contrails may be visible for only a few seconds or minutes, or may persist for many hours which may affect climate.
Contrails tend to last longer if there is higher moisture in the atmosphere and associated higher level clouds such as cirrus, cirrostratus and cirrocumulus already present before the plane flies through.
Condensation from engine exhaust
The main products of hydrocarbon fuel combustionCombustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
are carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and water vapour. At high altitudes this water vapour emerges into a cold environment, and the local increase in water vapour can push the water content of the air past saturation point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
. The vapour then condenses into tiny water droplets and/or deposits into ice. These millions of tiny water droplets and/or ice crystals form the contrails. The vapour's need to condense accounts for the contrail forming some way behind the aircraft's engines. At high altitudes, supercooled water vapour requires a trigger to encourage deposition or condensation. The exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapour to rapidly turn to ice crystals. Exhaust contrails usually occur above 8000 metres (26,000 feet), and only if the temperature there is below -40 °C.
Condensation from decreases in pressure
As a wing generates liftLift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
, it causes a vortex
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...
to form at each wingtip, and sometimes also at the tip of each wing flap
Flap (aircraft)
Flaps are normally hinged surfaces mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed an aircraft can be safely flown at and to increase the angle of descent for landing without increasing air speed. They shorten takeoff and landing distances as well as...
. These wingtip vortices
Wingtip vortices
Wingtip vortices are tubes of circulating air that are left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure...
persist in the atmosphere long after the aircraft has passed. The reduction in pressure and temperature across each vortex can cause water to condense and make the cores of the wingtip vortices visible. This effect is more common on humid days. Wingtip vortices can sometimes be seen behind the wing flaps of airliners during takeoff and landing, and during landing of the Space shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
.
The visible cores of wingtip vortices contrast with the other major type of contrails which are caused by the combustion of fuel. Contrails produced from jet engine exhaust are seen at high altitude, directly behind each engine. By contrast, the visible cores of wingtip vortices are usually seen only at low altitude where the aircraft is travelling slowly after takeoff or before landing, and where the ambient humidity is higher. They trail behind the wingtips and wing flaps rather than behind the engines.
During high-thrust settings the fan blades at the intake of a turbo-fan engine reach transonic speeds, causing a sudden drop in air pressure. This creates the condensation fog (inside the intake) which is often observed by air travelers during takeoff. For more information see the Prandtl-Glauert singularity
Prandtl-Glauert singularity
The Prandtl–Glauert singularity is the prediction by the Prandtl–Glauert transformation that infinite pressure conditions would be experienced by an aircraft as it approaches the speed of sound. Because it is invalid to apply the transformation at these speeds, the predicted singularity does not...
effect.
Contrails and climate
Contrails, by affecting the Earth's radiation balance, act as a radiative forcingRadiative forcing
In climate science, radiative forcing is generally defined as the change in net irradiance between different layers of the atmosphere. Typically, radiative forcing is quantified at the tropopause in units of watts per square meter. A positive forcing tends to warm the system, while a negative...
. Studies have found that contrails trap outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) at a greater rate than they reflect incoming solar radiation (negative radiative forcing). Global radiative forcing has been calculated from the reanalysis data, climatological models and radiative transfer codes. It is estimated to amount of 0.012 W/m2 for 2005, with an uncertainty range of 0.005 to 0.0026 W/m2, and with a low level of scientific understanding. Therefore, the overall net effect of contrails is positive, i.e. a warming effect. However, the effect varies daily and annually, and overall the magnitude of the forcing is not well known: globally (for 1992 air traffic conditions), values range from 3.5 mW/m2 to 17 mW/m2. Other studies have determined that night flights are mostly responsible for the warming effect: while accounting for only 25% of daily air traffic, they contribute 60 to 80% of contrail radiative forcing. Similarly, winter flights account for only 22% of annual air traffic, but contribute half of the annual mean radiative forcing.
September 11, 2001 climate impact study
The grounding of planes for three days in the United States after September 11, 2001 provided a rare opportunity for scientists to study the effects of contrails on climate forcing. Measurements showed that without contrails, the local diurnal temperature range (difference of day and night temperatures) was about 1 degree Celsius higher than immediately before; however, it has also been suggested that this was due to unusually clear weather during the period.Condensation trails have been suspected of causing "regional-scale surface temperature" changes for some time. Researcher David J. Travis, an atmospheric scientist at the University of Wisconsin-Whitewater
University of Wisconsin-Whitewater
The University of Wisconsin–Whitewater is part of the University of Wisconsin System, located in Whitewater, Wisconsin. It became Wisconsin's second public college on April 21, 1868 when it opened its doors to 39 students taught by nine faculty members...
, has published and spoken on the measurable impacts of contrails on climate change in the science journal Nature and at the American Meteorological Society
American Meteorological Society
The American Meteorological Society promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications. Founded in 1919, the American Meteorological Society has a membership...
10th Annual conference in Portland, Oregon. The effect of the change in aircraft contrail formation on the 3 days after the 11th was observed in surface temperature change, measured across over 4,000 reporting station
Weather station
A weather station is a facility, either on land or sea, with instruments and equipment for observing atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind...
s in the continental United States. Travis' research documented an "anomalous increase in the average diurnal temperature change". The diurnal temperature range
Diurnal temperature variation
Diurnal temperature variation is a meteorological term that relates to the variation in temperature that occurs from the highs of the day to the cool of nights.-Temperature lag:Temperature lag is an important factor in diurnal temperature variation...
(DTR) is the difference in the day's highs and lows at any weather reporting station. Travis observed a 1.8 degree Celsius departure from the two adjacent three-day periods to the 11th-14th. This increase was the largest recorded in 30 years, more than "2 standard deviations away from the mean DTR".
Head-on contrails
A contrail from an aeroplane flying towards the observer can appear to be generated by an object moving vertically. On 8 November 2010 in CaliforniaCalifornia
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, a contrail of this type gained wide media attention as a "mystery missile" that could not be explained by US military and aviation authorities, and its explanation as a contrail took more than 24 hours to become accepted by US media and military institutions.
Distrails
Distrails are created by the elevated temperature of the exhaust gases absorbing the moisture from the cloud. Clouds exist where the relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
is 100% but by increasing the temperature the air can hold more moisture and the relative humidity drops below 100%, even for the same absolute moisture density, causing the visible water droplets in the cloud to be converted back into water vapour.
External links
- Contrails.nl: Pictures of Contrails and Aviation Cirrus (-Smog), from 1995 on.
- Abstract of article in Nature announcing research results of contrail temperature change study
- Clouds Caused By Aircraft Exhaust May Warm The U.S. Climate
- Contrails over the USA
- Effects of contrails on ground astronomy
- Contrail simulator (Java applet) – interactively shows how temperature and humidity of the surrounding air affect contrail formation and characteristics
- Contrails: What's Left Behind Is Bad News, article by Nick Onkow from March 4, 2006
- NASA Contrail Education
- What is a contrail and how does it form?, National Weather ServiceNational Weather ServiceThe National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
- Vapor Trails March 1943 article